When looking around for a 40m diople, found this and built it I can say it works even if short! I got a good match with my MFJ analyser and on QRP, worked around Europe ok. I also heard VK/ZL! And only at 30 feet as an inv Vee so if stuck for space try this.
To construct each side of the antenna proceed as follows. Cut a 10.25 metre length of 24/0.076 insulated wire, and a 160 mm length of 40 mm o.d plastic tubing ( white plumbers tubing). Measure a 2.75 portion of the wire and attach the wire to the plastic former. Wind 40 turns of the wire onto the plastic former, and firmly secure the end of the winding. Make the other half of the antenna in the same way. Attach the ends of the 2.57 metre sections to a suitable centre insulator, which should also mount the choke balun, connect the 50 ohm coax, then carefully waterproof the whole assembly.
The choke balun uses RG174AU coax and a 40mm Ferrite Toriod.
Once the antenna is erected adjust it to resonance on 7.030Mhz (or wherever your fav 40m freg is!) by folding back the ends, and adjust the length to provide minimum SWR.

This could be made multiband by replacing the feed point with a 4:1 balun and feed with 50 Ω co-ax or directly with 300 Ω (or 450Ω) ribbon feeder to a balanced AMU or 4:1 then AMU. (update 8-09-10)
Article from SPRAT Issue 74 Spring 1993